A power over Ethernet (POE) describes any of several standardized or ad-hoc systems which pass electrical power along with data on Ethernet cabling. This allows a single cable to provide both data connection and electrical power to devices. Unlike standards such as an universal serial bus (USB) which also power devices over the data cables, the POE allows long cable lengths. In addition to standardizing existing practice for spare-pair and common-mode data pair power transmission, the POE standards provide signaling between an Ethernet power sourcing equipment (PSE) and a powered device (PD).
It is unnecessary for a PD to obtain power from an adapter, depending on the PD that is connected to the Ethernet PSE. However, at this moment a direct current (DC) PSE is necessary in response the PD is read as abnormal. The PD is connected to the Ethernet PSE or the DC PSE through different interfaces, because the function of a power circuit connected to the Ethernet PSE of the PD is different from the function of a power circuit connected to the DC PSE of the PD. The power circuit connected to the Ethernet PSE and the power circuit connected to the DC PSE also must be connected to different interfaces. Therefore, a single interface of the PD fed power is needed.